I would do something like this:

with basedata as (

select

Customer,

dateadd(day,[Date],'20040630') SalesDate,

Sales

from

dbo.YourTable

)

select

Customer,

dateadd(month,datediff(month,0,SalesDate),0) CalendarMonth,

sum(Sales) MonthlySales

from

basedata

group by

Customer,

dateadd(month,datediff(month,0,SalesDate),0)

order by

Customer,

dateadd(month,datediff(month,0,SalesDate),0);